Ion conducting nanofiber fuel cell electrodes

ABSTRACT

The present invention is directed to methods of making a nanofiber-nanoparticle network to be used as electrodes of fuel cells. The method comprises electrospinning a polymer-containing material on a substrate to form nanofibers and electrospraying a catalyst-containing material on the nanofibers on the same substrate. The nanofiber-nanoparticle network made by the methods is suitable for use as electrodes in fuel cells.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electrodes for fuel cells. Inparticular, it is directed to a process for manufacturing ananofiber-nanoparticle network that is useful for the manufacture ofelectrodes for use in fuel cells, to electrodes made by this process andto fuel cells incorporating such electrodes.

2. Description of the Related Technology

Fuel cells provide several advantages over batteries, such as highefficiency, high energy and power density, low weight, low-temperatureoperation, rapid start-up time, and quick fuels from renewable sourceswith no point-of-use greenhouse gas emissions. However, one major factorthat has limited the mass commercialization of fuel cells, especiallyfuel cell vehicles, is the high cost due to the catalysts requires foruse in fuel cells. The catalyst is usually a precious metal catalystwhich currently contributes to over 30% of the fuel cell engine cost.

Technologies that can reduce the amount of precious metal catalystneeded (i.e. allow low catalyst loadings) while still providing goodperformance (e.g. high power density) are critical to the successfulcommercialization of fuel cells. Because the catalyst functions on itssurface where it is in contact with a fuel, such as hydrogen or methane,the larger the surface area of the catalyst per unit weight, the lowerthe required catalyst loading. Extensive efforts have been investedtowards increasing the surface area of fuel cell catalysts.

U.S. Pat. No. 7,229,944 discloses a process of making fiber structuresbased on interconnected carbon fibers for use with catalytic material.The catalytic material may be in the form of nanosize particlessupported on the fibers. The structures are produced by electrospinninga polymeric material fiber structure that is subsequently converted to acarbon fiber structure in a heat treatment step which also causes thecatalyst particles to nucleate on the carbon fibers and grow to adesired nanosize. The catalyst may be uniformly distributed across thecarbon fiber structure before nucleation and the amount of catalyst maybe controlled. These factors may enhance catalytic performance and/orenable use of less catalyst for equivalent catalytic performance whichcan lead to cost savings, amongst other advantages.

U.S. Pat. No. 7,887,772 discloses an ultrafine graphitic carbon fiberhaving a diameter of 1 to 3000 nm that is prepared by electrospinning ahalogenated polymer solution containing a metal compound for inducinggraphitization. An ultrafine porous graphitic carbon fiber having alarge specific surface area, micropores and macropores is prepared bygraphitization using a metal catalyst generated from the metal compound.The ultrafine carbon fiber can be used for storing hydrogen, anadsorbing material for biochemically noxious substances, an electrodematerial for a supercapacitor, a secondary cell material, a fuel cellmaterial, or a catalyst carrier material.

U.S. Patent Application Publication No. US 2012/0028170 discloses a fuelcell electrode made by synthesizing carbon nanotubes grafted withpoly(citric acid) and encapsulating a platinum group metal nanoparticle.More specifically, carbon nanotubes are oxidized, followed by mixingwith monohydrated citric acid, which results in carbon nanotubes graftedwith poly(citric acid). The carbon nanotubes grafted with poly(citricacid) are then mixed with one or more sources of platinum group metalions to encapsulate the platinum group metal nanoparticles. Finally, thecarbon nanotubes encapsulated with platinum group metal nanoparticlesare electrosprayed onto an electrode of a fuel cell. The presentinvention is aimed at achieving high power densities in fuel cells withrelatively low catalyst loadings.

SUMMARY OF THE INVENTION

In a first aspect, the present invention is directed to a method formaking a nanofiber-nanoparticle network useful in an electrode for afuel cell. The method includes the steps of electrospraying acatalyst-containing material and electrospinning a polymer-containingmaterial to form a nanofiber-nanoparticle network.

Another aspect of the present invention is an electrode for fuel cellscomprising the nanofiber-nanoparticle network manufactured by the methodof the present invention.

Yet another aspect of the present invention is a fuel cell that uses anelectrode comprising the nanofiber-nanoparticle network manufactured bythe method of the present invention.

Yet other aspects of the present invention relate to a method for makinga patterned nanofiber-nanoparticle network useful in an electrode for afuel cell, electrodes made with the patterned nanofiber-nanoparticlenetwork and fuel cells including such electrodes.

In a still further aspect of the present, catalyst materials for use infuel cells are provided, as well as electrodes and fuel cells includingthese catalyst materials. These catalysts may include platinum/carbidederived carbon (Pt/CDC).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow chart of a process for manufacturing ananofiber-nanoparticle network according to the present invention.

FIG. 2 shows another embodiment of a process for manufacturing ananofiber-nanoparticle network according to the present invention.

FIG. 3A shows a single Nafion nanofiber bridging two electrodes.

FIG. 3B shows an enlarged image of FIG. 3A.

FIG. 3C is a plot of the proton conductivity of a Nafion nanofiber vs.the nanofiber diameter.

FIG. 3D is a plot of the proton conductivity of a Nafion nanofiber vs.relative humidity.

FIG. 4A is a scanning electron microscope (SEM) image of ante mortemnanofiber-nanoparticle network manufactured according to the presentinvention.

FIG. 4B is an SEM image of post mortem nanofiber-nanoparticle networkmanufactured according to the present invention.

FIG. 5A is an SEM image of a commercial Pt/C catalyst.

FIG. 5B is an SEM image of a platinum/carbide derived carbon (Pt/CDC)supported catalyst according to the present invention.

FIG. 6 shows micro-patterned substrates that may be used in the presentinvention.

FIG. 7 shows a fuel cell including an electrode made by the process ofthe present invention.

FIG. 8 shows a diagram of the interface of three phases: catalyst,polyelectrolyte, and pores.

FIG. 9A is an SEM image of an electrode with 0.022 mg/cm² platinum (Pt)loading, fabricated according to the method of Example 4 according toone embodiment of the present invention.

FIG. 9B is an enlarged SEM image of the same electrode shown in FIG. 9A.

FIG. 9C is plot of the distribution of the diameters of nanofibers inthe same electrode shown in FIGS. 9A-9B.

FIG. 9D is a plot of the distribution of the sizes of the nanoparticlesin the same electrode shown in FIGS. 9A-9B.

FIG. 9E is an SEM image of an electrode with 0.052 mg/cm² Pt loading,fabricated by the method of Example 4 according to one embodiment of thepresent invention.

FIG. 9F is an enlarged SEM image of the electrode shown in FIG. 9E.

FIG. 9G is plot showing the distribution of the diameters of nanofibersin the electrode shown in FIG. 9E.

FIG. 9H is a plot showing the distribution of the sizes of nanoparticlesin the electrode shown in FIG. 9E.

FIG. 10A shows performance of fuel cells with the electrodes fabricatedin Example 4 and comparative hand-painted control electrodes withoperating conditions of H₂/air at ambient pressure.

FIG. 10B shows performance of the same fuel cells used in FIG. 10A, atoperating conditions using H₂/air with 25 psi back pressure.

FIG. 10C shows performance of the same fuel cells used in FIG. 10A, atoperating conditions using H₂/O₂ at ambient pressure.

FIG. 10D shows performance of the same fuel cells used in FIG. 10A, atoperating conditions using H₂/O₂ with 25 psi back pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

For illustrative purposes, the principles of the present invention aredescribed by referencing various exemplary embodiments. Although certainembodiments of the invention are specifically described herein, one ofordinary skill in the art will readily recognize that the sameprinciples are equally applicable to, and can be employed in othersystems and methods. Before explaining the disclosed embodiments of thepresent invention in detail, it is to be understood that the inventionis not limited in its application to the details of any particularembodiment shown. Additionally, the terminology used herein is for thepurpose of description and not of limitation. Furthermore, althoughcertain methods are described with reference to steps that are presentedherein in a certain order, in many instances, these steps may beperformed in any order as may be appreciated by one skilled in the art;the novel method is therefore not limited to the particular arrangementof steps disclosed herein.

It must be noted that as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural references unless thecontext clearly dictates otherwise. Furthermore, the terms “a” (or“an”), “one or more” and “at least one” can be used interchangeablyherein. The terms “comprising”, “including”, “having” and “constructedfrom” can also be used interchangeably.

In a first aspect, the present invention relates to a novel process formanufacturing a nanofiber-nanoparticle network useful for fuel cellelectrodes. The process comprises two steps: electrospinning of apolymer-containing material; and electrospraying of acatalyst-containing material onto the electrospun polymer. Theelectrospinning and electrospraying may be carried out in any suitablemanner. Some examples include electrospinning and electrosprayingsimultaneously, or first electrospinning and then electrospraying.

Electrodes made in this manner have more catalyst surface area availablefor an oxygen reduction reaction because a large percentage of catalystin such electrodes can directly contact the fuel in the fuel cell. Suchelectrodes therefore require less catalyst than electrodes usingconventional supported catalysts, thereby permitting low or ultralowcatalyst loadings. In addition, these catalysts are more efficient foroxidization of the fuel, i.e. provide a higher power density thanconventional fuel cell electrodes

Referring to FIG. 1, step 1 of the process may involve preparation of apolymer-containing material. The polymer-containing material contains atleast one polymer, preferably Nafion. However, any suitable polymer maybe used to make the nanofiber material. In general, the polymer needs tohave properties that make it possible to fabricate a nanofiber supportusing electrospinning. In addition, the polymer may have a protonconductivity of from about 0.001 mS/cm to about 10 S/cm, or from about0.1 mS/cm to about 1 S/cm, or from about 0.1 S/cm to about 1 S/cm.Suitable polymer materials include, but are not limited to, Nafion,sulfonated poly(ether ether ketone), sulfonatedpolyer(styrene-b-ethylene-r-butadiene-b-styrene), sulfonatedpoly(styrene), sulfonated poly(arylene ether) copolymer, sulfonatedpoly(styrene-b-isobutylene-b-styrene).

In one embodiment, the polymer may be dissolved in a suitable solvent toprovide a polymer solution for electrospinning Suitable solvents areknown to those of skill in the art and their suitability depends, atleast in part, on the characteristics of the polymer. Suitable solventsmay include, for example, N,N-dimethylformamide (DMF), ethanol,methanol, acetone, water, tetrahydrofuran (THF), methylene chloride (MCor dichloromethane) and combinations thereof. It should be understoodthat a skilled person in the art may also choose other solvents that aresuitable for a particular process.

The concentration of the polymer in the polymer solution used forelectrospinning can be determined by a skilled person based on, forexample, the desired viscosity of the polymer solution. Typical polymerconcentrations may be between about 8% and about 20% by weight of thesolution, and more preferably between about 8% and about 15% by weightof the solution. Such concentrations generally result in the solutionhaving a suitable viscosity for electrospinning. It should be understoodthat concentrations outside the above ranges may also be used if theresultant polymer solution is suitable for electrospinning.

In another embodiment, the polymer may be melted, preferably by heating,to make a liquid polymer having a suitable viscosity forelectrospinning. Depending on the characteristics of the polymer used,it may be preferred to melt the polymer, instead of using a solvent toprovide a polymer solution. When a melted polymer is used as theelectrospinning material, the melted polymer may need to be maintainedat elevated temperature for electrospinning. Thus, some modification ofthe electrospinning apparatus may be needed to accommodate this heatingrequirement, such as providing a heated electrospinning needle/nozzle.

In step 2 of FIG. 1, the polymer-containing materials supplied to atleast one electrospinning needle/nozzle (hereinafter needle/nozzle isreferred to as “needle”). In a preferred embodiment, thepolymer-containing material is pumped to the electrospinning needle. Anyother suitable means for supplying the polymer-containing material tothe electrospinning needle may be used in the present invention.

In step 3 of FIG. 1, the electrospinning needle electrospins thepolymer-containing material onto a grounded substrate as a fine streamwith its diameter in the nanometer range. In a typical apparatus forelectrospinning, the electrospinning needle is connected to a highvoltage power supply, while the conductive substrate is grounded. Thusan electromagnetic field is formed between the electrospinning needleand the grounded substrate. The polymer-containing material, aided bythe force of the electromagnetic field, travels to the substrate byelectrostatic attraction.

The voltage applied to the electrospinning needle may depend on thepolymer and the viscosity of the polymer-containing material preparedfrom the polymer. In an exemplary embodiment, the voltage applied to theelectrospinning needle is between about 3 kV and about 50 kV, or betweenabout 10 kV and about 40 kV.

The distance between the tip of the electrospinning needle and thesubstrate may depend on the diameter of the polymer-containing materialstream, as well as the viscosity of the polymer-containing material. Ingeneral, a finer stream and lower viscosity may require a shorterdistance between the electrospinning needle and the substrate. In someexemplary embodiments, the distance between the tip of theelectrospinning needle and the substrate may be between about 1 cm andabout 50 cm, more preferably between about 5 cm and 35 cm.

The speed of electrospinning may depend on the polymer. Generallyspeaking, the faster the polymer-containing material solidifies ordries, the higher the speed of electrospinning that may be used. Thepolymer material may be dispensed for example at about 0.1 to about 10mL/hour) through the electrospinning needle.

In an exemplary embodiment, one electrospinning needle is used to spinthe polymer-containing material onto the substrate. However, in someembodiments, it may be desirable to use multiple electrospinningneedles. A general description of an electrospinning process isprovided, for example, in “Polymer Nanofibers Assembled byElectrospinning”, Frenot et. al, Current Opinion in Colloid andInterface Science, vol. 8, pages 64-75, 2003, which is incorporatedherein by reference in its entirety.

In some embodiments, the polymer-containing material may form acontinuous nanofiber, or the nanofiber may break and a plurality ofseparate nanofibers may form on the substrate.

In step 4 of FIG. 1, a catalyst-containing material suitable forelectrospraying is prepared. In some embodiments, thecatalyst-containing material is a solution that contains at least onecatalyst that can function as an oxidation/reduction catalyst in a fuelcell. The catalyst may be selected from, but is not limited to,palladium, platinum, gold, silver, nickel, rhodium, ruthenium, rhenium,osmium, iridium, iron, chromium, cobalt, copper, manganese, tungsten,niobium, titanium, tantalum, lead, indium, cadmium, tin, bismuth,gallium, as well as mixtures, compounds and alloys of these metals. Insome embodiments, palladium and platinum may be the preferred catalysts.

The present invention may also employ new catalysts for fuel cells.These catalysts include platinum/carbide derived carbon (Pt/CDC). Theplatinum on CDC supports (FIG. 5B) has a higher nanoporosity than acomparable platinum on amorphous carbon catalyst (FIG. 5A). Therefore,higher platinum surface area to volume ratios can be achieved with thePt/CDC catalyst. Examples of such catalysts can be found, for example,in US 2010/0285392 A1.

In step 5 of FIG. 1, the catalyst-containing material is supplied to theelectrospraying nozzle or needle (hereinafter collectively referred toas a “needle”). In one embodiment, the catalyst-containing material ispumped to the electrospraying needle. Any other means known to personsskilled in the art that are capable of supplying the catalyst-containingmaterial to the needle may be used in the present invention.

In step 6 of FIG. 1, the electrospraying needle sprays thecatalyst-containing material as fine droplets onto the electrospunmaterial or the formed nanofibers. The sprayed fine droplets containingcatalyst material are electrostatically attracted to the substrate asdiscussed below by electrostatic attraction. For the present invention,the same high voltage power supply may be connected to both theelectrospray needle and the electrospinning needle, or they may beconnected to different high voltage power supplies. In one exemplaryembodiment, the voltage applied to the electrospraying needle is betweenabout 3 kV and about 45 kV.

The distance between the tip of the electrospray needle and thesubstrate may depend on the size of the sprayed droplets, as well as theviscosity of the sprayed catalyst-containing material. In general, thesmaller the size of the sprayed material droplets and lower viscositywill require shorter distance between the electrospray needle and thesubstrate. In some exemplary embodiments, the distance may be betweenabout 1 cm and about 50 cm, more preferably between about 3 cm and 30cm.

The speed of electrospraying of the catalyst-containing material maydepend on the speed at which the nanofibers are formed on the substrateand the desired nanoparticle density on the nanofibers. For example, thecatalyst material may be sprayed at about 0.01 to 50 mL/hour.

In an exemplary embodiment, one electrospraying needle is used to spraythe catalyst-containing material onto the nanofibers on the substrate.However, in some embodiments, it may be desirable to use multipleelectrospray needles.

The substrate onto which the catalyst-containing droplets areelectrosprayed and polymer-containing fine streams are electrospun isconductive such that, when grounded, there is an electromagnetic fieldformed between the substrate and electrospinning needle/electrosprayneedle. The substrate can be made from any suitable conductive material.In preferred embodiments, the substrate is made of a metal, such as, forexample, aluminum (Al).

In some exemplary embodiments, the substrate may be patterned ormicro-patterned as shown in FIG. 6 to provide control over the thenanofiber morphology and provide an improved fuel cell performance evenat low catalyst loadings. Micro-patterned substrates are disclosed inZhang and Chang, “Electrospinning of three-dimensional nanofibrous tubeswith controllable architectures,” Nano Letters, vol. 8, no. 10, pages3283-3287, 2008), which is incorporated herein in its entirety byreference. The use of a patterned substrate allows at least some or allof the nanofibers in the nanofiber/nanoparticle network to be aligned ina substantially parallel relationship relative to one another.

In step 7 of FIG. 1, the nanofiber-nanoparticle network is formed. Ifthe polymer-containing material is a solution, the solvent evaporates tosolidify the polymer and form nanofibers on the substrate. If moltenpolymer is employed, loss of heat from the polymer-containing materialwill form nanofibers on the substrate.

The nanofibers generally have circular-shaped cross-sections, thoughother cross-sections may also be suitable. In some embodiments, thenanofibers are preferably solid (i.e., not hollow). However, it shouldbe understood that in other embodiments, the fibers may be hollow atleast at some sections of the nanofibers (e.g., nanotubes).

The nanofibers can have any suitable dimension. In some embodiments, theaverage nanofiber diameter is greater than about 10 nm; in someembodiments, greater than about 50 nm; and, in some embodiments, greaterthan about 100 nm. Nanofiber diameters less than these ranges may causethe structure to have insufficient mechanical integrity for somepolymers used. The nanofiber diameters may be less than about 1 micron,more preferably less than about 500 nm. In some embodiments, thenanofiber diameters may be less than about 300 nm.

The length of the nanofibers may vary. In some embodiments, one or morenanofibers may have a length of at least about 500 microns; and, in someembodiments, at least one nanofiber has a length greater than about 1mm. In some embodiments, it may be possible to achieve nanofiber lengthsof greater than about 1 cm, or even significantly greater.

Forming of the nanofiber-nanoparticle network of step 7 also includesforming or depositing of catalyst nanoparticles on the nanofibers. Thefine droplets containing catalyst material will dry by loss of solvent.The loss of solvent results in the formation of catalyst nanoparticleson the nanofibers. In some embodiments heat treatment may be needed tofacilitate or accelerate the formation of nanoparticles.

The catalyst-containing nanoparticles are preferably substantiallyevenly distributed on the surface of nanofibers, though some aggregationof nanoparticles to aggregates of up to about 0.5 microns is generallyacceptable. The nanoparticle size is generally in the nanometer range.For example, the nanoparticles may have an average particle size of lessthan about 50 nm; and, in some cases, less than about 20 nm. Smallnanoparticle sizes may advantageously lead to the relatively uniformdistribution of catalytic material throughout the nanofibers, as well aslarger surface areas for oxidation, amongst other positive effects.However, the nanoparticles typically (though not always) have an averageparticle size of greater than 20 nm. It should be understood thatnanoparticle sizes outside the above ranges may be used in certainembodiments of the present invention.

Average nanoparticle sizes may be determined by averaging thenanoparticle sizes of a representative number of nanoparticles using,for example, scanning electron microscope (SEM) techniques. As usedherein, the average nanoparticle size includes sizes of primarynanoparticles and sizes of nanoparticle agglomerates. It may bepreferred for the nanoparticle size distribution to be relativelynarrow, and/or relatively homogenously distributed. Narrow nanoparticlesize distributions promote the uniform distribution of catalyticmaterial throughout the nanofiber-nanoparticle network.

During step 7 of FIG. 1, for the purpose of forming thenanofiber-nanoparticle network, heat may be applied to the materialsdeposited on the surface of the substrate. Heat may be introduced to thedeposited nanofiber material and nanoparticle material by heating thesubstrate itself, or by some other suitable means such as heated air orinfra-red radiation. The heat may accelerate the evaporation of solvent.Some further desirable transformations and/or reactions may also beinitiated or accelerated by heating.

In a preferred embodiment of the present invention, the speed ofelectrospinning of nanofiber material and the speed of electrosprayingof the nanoparticle material are controlled relative to one another. Theoptimal speed of electrospinning and optimal speed of electrosprayingmay be determined by a skilled person.

The movement of the electrospinning needle and/or the movement of thesubstrate are also preferably controlled to ensure that the nanofibersare substantially evenly distributed over at least portion of thesurface of the substrate. In addition, the motion of the electrospinningneedle and the electrospray needle are also preferably controlled toensure that catalyst nanoparticles are evenly deposited on substantiallyall of the nanofibers.

In some exemplary embodiments, the substrate may be kept stationary,while both the electrospinning needle and electrospray needle move inthe space over the substrate to spread the nanofibers and nanoparticleson the surface of the substrate. In one embodiment, the stationarysubstrate may be cylindrical. The electrospinning needle andelectrospray needle may both rotate around the cylindrical substrate. Inyet another embodiment, the stationary substrate may be flat. Theelectrospinning needle and electrospraying needle may both travel fromone end of the flat substrate to the other end.

Referring to FIG. 2, in an exemplary embodiment, the process of thepresent invention is carried out in a closed chamber 10. The chamber 10has an air inlet 1 and an air outlet 8. A tube 2 supplies acatalyst-containing material to an electrospraying needle 5, which isconnected to a high voltage power supply by a wire 3. The substrate, arotating collector 6 is grounded through a wire 9. A tube 4 supplies thepolymer-containing material to electrospinning needle 7, which isconnected to a high voltage power supply through a wire 3.

In this embodiment, electrospinning and electrospraying are carried outin any suitable manner. Because the collector 6 rotates, the nanofibersare substantially evenly distributed on the surface of the collector 6,and the nanoparticles are substantially evenly distributed on thenanofibers.

In certain embodiments, the process of the present invention may furthercomprise a treatment step. The treatment may be initiated either duringor after the formation step 7 in FIG. 1. One such treatment step is thefusion of at least portion of the nanofibers at points where thenanofibers intersect. The nanofibers may be fused together during thebeginning stage of heat treatment. This fusion may lead to increasedmechanical integrity and/or increased conductivity. Some of thenanofibers may be merely in physical contact with one another atintersection points without being fused together.

The nanofiber-nanoparticle network made by the method depicted in FIG. 1is suitable for use as an electrode in fuel cells. The conductivesubstrate used for electrospinning/electrospraying may be removed fromthe nanofiber-nanoparticle network before the nanofiber-nanoparticlenetwork is used as an electrode. FIG. 7 shows an exemplary embodiment ofa hydrogen fuel cell using electrodes with a nanofiber-nanoparticlenetwork made by the process described in the present invention. In thisexemplary fuel cell, the electrolyte is made from Nafion.

One advantage of the nanofiber-nanoparticle network made by the methoddepicted in FIG. 1 is the availability of a larger surface area per unitof catalyst, where the reaction of the fuel cells us carried out, asshown in FIG. 8. For example, the electrodes embodying thenanofiber-nanoparticle network may be porous electrodes consisting of,for example, platinum (Pt) as catalyst in the form of nanoparticles,Nafion polymer, and pores. Hydrogen and oxygen react on the Pt surfaceand protons are transported through the Nafion polymer connected thenetwork. Nafion acts as both a binder and a proton transporter and thepores serve as channels for gas diffusion to the catalyst surface.

The electrodes of the present invention may have catalyst loading of,for example, less than 0.1 mgPt/cm², more preferably, less than 0.05mgPt/cm², and most preferably, less than 0.01 mgPt/cm². It has beenfound that these relatively low catalyst loadings for electrodes ofpresent invention provided power densities comparable to electrodeshaving a standard loading of 0.4 mgPt/cm², (0.77 gPt/kW, 0.522 W/cm²),which electrodes were made by a standard electrode fabricationtechnique. Electrodes made by the present invention may have a Ptloading/power of at least 0.1 gPt/kW, more preferably, at least 0.06gPt/kW. This is much lower than the 2015 Department of Energy target of0.15 gPt/kW.

The invention will now be illustrated by the following examples whichare not to be construed as limiting of the invention.

EXAMPLES Example 1

A nanofiber-nanoparticle network made by the process according to thepresent invention was used in electrodes for a fuel cell. Theseelectrodes used platinum as the catalyst. It was observed that theseelectrodes had a high fuel cell power density at Pt loadings that are 40times lower than typical Pt loadings used in state-of-the-art fuelcells.

More specifically, a 0.01 mgPt/cm² (0.158 W/cm², 0.06 gPt/kW) loadingfor electrodes of present invention provided power densities comparableto the standard catalyst loading of 0.4 mgPt/cm², (0.77 gPt/kW, 0.522W/cm²) of electrodes made by a standard electrode fabrication technique.It was also observed that the Pt loading/power was 0.06 gPt/kW for theelectrode of the present invention, which was much lower than the 2015Department of Energy target of 0.15 gPt/kW.

Example 2

Nanofibers made from Nafion were found to have extremely high protonconductivity. FIG. 3A, is an SEM image of a single Nafion nanofiberbridging the gap between two metal electrodes. FIG. 3B is an enlargedimage of FIG. 3A. The Nafion nanofiber's proton conductivities at 30° C.and 90% relative humidity as a function of nanofiber diameter are shownin FIG. 3C. The proton conductivities of nanofibers with diameters >2 μmwas similar to that of bulk Nafion film (˜0.1 S/cm). However, when thenanofiber diameter was <1 μm, proton conductivity increased sharply withdecreasing nanofiber diameter and reached a value as high as 1.5 S/cmfor the 400 nm nanofiber, which was an order of magnitude higher thanthe bulk Nafion film. The relative humidity also affected the protonconductivity of the Nafion nanofiber, as shown in FIG. 3D. See alsoDong, B.; Gwee, L.; Salas-de la Cruz, D.; Winey, K. I. Elabd, Y. A.Super Proton Conductive High Purity Nafion Nanofibers. Nano Letters2010, 10, 3785-3790.

Example 3

The ion-conducting nanofiber-nanoparticle network manufactured accordingto one embodiment of the present invention is shown in FIGS. 4A and 4B.This network is ideal for use as a fuel cell electrode. SEM images ofthe electrode ante mortem (FIG. 4A) and post mortem (FIG. 4B)demonstrate the nanofiber morphology, where catalyst nanoparticles aresubstantially evenly distributed on the surface of the nanofibers createan electrode with high surface area. Such electrodes have a low catalystloading and a high power density.

Example 4

A custom-designed apparatus for producing electrodes according to theelectrospinning/electrospraying (E/E) process of the present inventionwas used to fabricate electrodes in this example. The apparatusconsisted of two high-voltage power supplies (Model PS/EL50R00.8 ofGlassman High Voltage, Inc. and Model ES40P-10W/DAM of Gama High VoltageResearch), two syringe pumps (Model NE-1000, New Era Pump Systems), twosyringe needles (i.d.=0.024 in.) (Hamilton), tubing (Pt No. 30600-65,Cole-Parmer), and a grounded collector (aluminum foil coated drum,o.d.=4.85 cm). The collector drum was connected to a motor (Model4IK25GN-SW2, Oriental Motor) to allow for rotation during the E/Eprocess. A gas diffusion layer was adhered to the collector drum, wherenanofibers/nanoparticles were directly collected via the E/E process.The needle tip to collector distances, applied voltages, and solutionflow rates were 15 cm and 9 cm, 10.5 kV and 12.5 kV, and 0.3 ml/h and 3ml/h for the E/E process, respectively. As a comparison, a hand-paintingprocess was used to make hand-painted electrodes as a control.

The materials used to fabricate the electrodes included de-ionized (DI)water, isopropanol (IPA, Sigma-Aldrich, 99.5%), ethanol (Decon Labs,Inc., 99.5%), and 5 wt % Nafion in a water/isopropanol solution (1000EW, Ion Power), poly(acrylic acid) (PAA) (M_(v)=450,000 g/mol, Aldrich),20 wt % Pt on carbon catalyst (Vulcan XC-72, Premetek Co.), and aSGL-25BC gas diffusion layer (Fuel Cells Etc.). These materials wereused to fabricate the electrodes in both the hand-painting process (tofabricate hand-painted electrodes) and the E/E process of the presentinvention (to fabricate E/E electrodes).

Catalyst ink used in the electrospraying portion of the E/E processaccording to the present invention consisted of 20 mg platinum (Pt)catalyst, 0.248 ml DI water, 0.043 ml 5 wt % Nafion solution, 0.171 mlIPA/H₂O (3/1 v/v), and 1.970 ml ethanol. This mixture was sonicated for3 min (Model CL-18, Qsonica Sonicator) prior to electrospraying. Thismixture corresponds to 10/1 wt/wt (Pt/C)/Nafion, which was five timesgreater than the same ratio in the catalyst ink prepared for thehand-painted electrodes. Significantly less Nafion was required forelectrospraying, because Nafion was also supplied from theelectrospinning to produce the E/E electrode. The final (Pt/C)/Nafionratio in the E/E electrode was similar to that of the hand-paintedelectrode, since Nafion was supply to electrodes from bothelectrospraying and electrospinning

The polymer solution used in the electrospinning portion of the E/Eprocess consisted of a mixture of 4/1 wt/wt Nafion/PAA. A solution of 5wt % PAA and Nafion was prepared by combining 131.1 mg PAA, 10494 mg of5 wt % Nafion solution, and 2491.7 mg IPA/water (3/1 v/v). This solutionwas stirred at 70-80° C. for ˜12 h to ensure complete dissolution. Thesolution was cooled down to ambient temperature before electrospinningThe electrodes were annealed at 135° C. for 5 min to stabilize thenanoparticles and nanofibers right after the E/E process. For the E/Eelectrodes, the Pt loading was varied by changing the E/E process timefor making a particular electrode.

The catalyst ink used to make hand-painted electrodes was prepared bycombining 100 mg solid Pt catalyst, 550 mg DI water, 1000 mg 5 wt %Nafion solution, and 1350 mg IPA and mixing via sonication for 3 min.This mixture corresponds to 2/1 wt/wt (Pt/C)/Nafion and 3/1 v/vIPA/water.

Membrane electrode assemblies (MEA's) were made by sandwiching togetherthe hand-painted electrodes or E/E electrodes and NR-212 Nafionmembrane. The anode catalyst layers of all of the membrane electrodeassemblies were hand-painted with a Pt loading of 0.15 mg/cm². All ofthe membrane electrode assemblies were hot-pressed at 135° C., 33 psifor 5 minutes.

Example 5

The catalyst loading of the electrodes fabricated in Example 4 wasdetermined by thermogravimeteric analysis (hereinafter “TGA”) (TGA 7,Perkin Elmer). A small portion of the E/E electrode (˜5-7 mg) was heatedduring the TGA from ambient temperature to 900° C. at 5° C./min in airflowing at 20 ml/min. Since all components in the E/E electrodevolatilize above 900° C., with the exception of Pt, the Pt loading wasdetermined by comparing the weight of the E/E electrode before and afterexposure to a temperature of 900° C. in the TGA.

Morphological characterization of the E/E electrodes fabricated inExample 4 was carried out with scanning electron microscopy (SEM, ModelFEI/Philips XL-30, 10 kV). SEM images of the E/E electrode were obtainedafter the E/E process (on a gas diffusion layer) and before MEAfabrication. All samples were sputter-coated (Denton Desk II SputteringSystem) with platinum at 40 mA for 30 seconds before the SEM images weretaken.

SEM images of the E/E electrode with 0.022 mg/cm² Pt loading are shownin FIGS. 9A and 9B. FIG. 9A shows that the E/E electrode was porous anduniform over large length scales (˜10 μm). The porous structure allowsgas transport from the gas flow channel to the catalyst layer. TheNafion/PAA nanofibers serve as pathways for proton transport, while theplatinum/carbon (Pt/C) nanoparticles are the active sites forelectrochemical reactions. FIG. 9B shows a magnified view of the E/Eelectrode of FIG. 9A, where several large agglomerates (˜1-2 μm) wereobserved, while the majority of catalyst particles ranged in size from˜50-300 nm. The larger agglomerates were rough and porous, which stillallow for gas/fuel to penetrate and diffusion for electrochemicalreaction. When the E/E electrode was compressed, the nanoparticles andnanofibers formed a tightly packed network for both electron and protontransport.

Using SEM, the size distributions of the nanoparticles and nanofibers inthe E/E electrodes were also studied. Thirty of the nanofibers shown inFIG. 9B were randomly selected using ImageJ software for determinationof nanofiber diameter by SEM. FIG. 9C presents the distribution of thediameters of the thirty randomly selected nanofibers. It was observedthat the majority of the nanofibers had diameters of ˜200 nm.

The sizes of the same nanoparticles randomly selected from FIG. 9B wasalso determined by SEM. FIG. 9D shows the distribution of nanoparticlesize (diameter) in the E/E electrode. The size distribution of thenanoparticles was broader than that of the nanofibers, with ˜80% of thenanoparticles being less than 450 nm in diameter.

A similar morphological study was also conducted on an E/E electrodewith a higher Pt loading (0.052 mg/cm²), which was also fabricated inExample 4 (see FIGS. 9E-9H). Surprisingly, more catalyst agglomerateswere seen in FIG. 9E than for the E/E electrode with the lower Ptloading shown in FIG. 9A. This may be due to a slight change in theambient temperature and relative humidity that may have affected theviscosity and evaporation rate of the solvent in the catalyst ink. Thedistributions of nanofiber diameters and nanoparticle sizes in FIGS. 9Gand 9H show that that the nanofiber sizes were similar to those shown inFIG. 9C, but that the nanoparticle sizes were on average larger thanthose in FIG. 9D. This suggests that, though a change in ambientconditions may affect nanoparticle size, it may not have a significanteffect on nanofiber diameters.

Additionally, it was observed that the electrosprayed nanoparticles weremore sensitive to slight changes in operating conditions. The flow rateof the electrospraying process (3 ml/h) was higher than the flow rate ofthe electrospinning process (0.3 ml/h). The higher flow rate of theelectrospraying relative to the electrospinning was selected to achievea desired ratio of Pt/C to Nafion (2:1) in the catalyst layer. Thehigher flow rate of the electrospraying solution required a longer timeto fully evaporate the solvent inside the nanoparticles. As a result, aslight change in operating conditions such as relative humidity andtemperature could significantly affect the morphology and size of thenanoparticles in the E/E electrodes, but appears to have a lesser effecton the diameter of the nanofibers.

Example 6

The membrane electrode assembly (1.21 cm², made in Example 4) was putinto fuel cells (as cathode) with 100 lb force torque. The membraneelectrode assembly was placed between two serpentine flow field graphiteplates separated by two 0.160 mm thick Teflon® coated gaskets (Pt No.381-6, Saint Gobian). A hand-painted cathode with a 0.42 mg/cm² Ptloading was used as a control. For the E/E cathode electrodes, the Ptloading had two levels: 0.052 and 0.022 mg/cm². Hand-painted electrodeswith 0.15 mg/cm² Pt loading were used as the anode in all fuel cells inthis Example.

The fuel cell performances were evaluated with a Compact Fuel Cell TestSystem (850C, Scribner Associates, Inc.). Fuel cell tests were conductedat ambient temperature and 25 psi back pressure with anode and cathodeflow rates of 0.42 L/min hydrogen and 1.0 L/min air, respectively. Thecathode, anode, and fuel cells were kept at 80° C. When H₂ and O₂ weresupplied to the anode and cathode, the flow rates were 0.42 L/min and0.50 L/min, respectively. Polarization curves were collected from opencircuit to 0.2 V at increments of 0.05 V/min.

The fuel cell performance was recorded after a new membrane electrodeassembly was fully activated. The activation process for the fuel cellsincluded operating a membrane electrode assembly at 0.7 V for 1-2 hoursfollowed by scanning the voltage from open circuit potential to 0.2 Vseveral times. This activation process lasted ˜4-6 h and may be repeateduntil the membrane electrode assembly reached steady state. A steadystate was reached when no further increase in the current was observedand the fuel cell was held at constant voltage. All fuel cellperformances were measured using fully saturated anode and cathodefeeding gases (Relative Humidity=100% for anode and cathode).

FIGS. 10A-10D show the performance of fuel cells with the E/E cathodesat both Pt loading levels, as well as fuel cells with the hand-paintedcathodes as control. FIG. 10A shows that the fuel cell performanceincreased with the Pt loading. The maximum power density of the controlexperiment (hand-painted cathode with 0.42 mg/cm² Pt loading) was 0.59W/cm², while the E/E cathodes with 0.052 and 0.022 mg/cm² Pt loadingsresulted in maximum power densities of 0.436 W/cm² and 0.376 W/cm²,respectively. When the Pt loading of the E/E electrode (0.052 mg/cm² Ptloading) was 8-fold lower than the hand-painted electrode (0.42 mg/cm²Pt loading), the output power was only reduced by 37%. The output powerof the E/E electrode with 0.022 mg/cm² Pt loading was 20-fold lower thanthe output power hand-painted control electrode, but this only resultedin a 46% reduction in the maximum output.

The effect of the catalyst loading on the fuel cell performance whenback pressure was applied to the fuel cells was also investigated. Underback pressure, the fuel cell performance was less affected by thecatalyst loading on the electrode (FIG. 10B), probably because backpressure led to a higher concentration of the reactants at the catalystsurface. When 25 psi back pressure (39.7 psi absolute pressure) wasapplied to both the cathode and anode in the fuel cells, the maximumoutput power for the control experiment (0.42 mg/cm² Pt loading on thehand-painted cathode) was 0.839 W/cm². For the E/E cathodes with 0.022mg/cm² and 0.052 mg/cm² Pt loading, the maximum output power was 0.625W/cm² and 0.656 W/cm², respectively. This corresponded to a reduction ofonly 26% and 22% in the maximum output power for the E/E electrodescompared to the hand-painted control electrode. Thus, the fuel cellswith E/E electrodes were capable of generating high power densities atultra-low Pt loadings (significantly lower cost than control).

FIG. 10C shows fuel cell performance with oxygen (O₂), instead of air,as the cathode fuel. It was observed that the fuel cell performance inthe high current region (mass transport-limiting region) improvedsignificantly in comparison with when air was used as the cathode fuel.This might be due to the fact that pure oxygen drives a more efficientelectrochemical reaction at the anode. An output power density of 0.962W/cm² was achieved when pure oxygen was supplied to the hand-paintedcathode in the control experiment. For the E/E cathodes withsignificantly lower Pt loadings than the control hand-painted cathode,the maximum power output was still high (0.724 W/cm² and 0.614 W/cm² forthe 0.052 mg/cm² and 0.022 mg²/cm Pt loadings, respectively). Thiscorresponded to only a 25-36% lower maximum output power densitycompared to the control experiment with an 8-20-fold reduction in Ptloading. When 25 psi back pressure was applied to both the cathode andanode sides, a further improvement in fuel cell performance was observed(FIG. 10D), with back pressure reducing the difference in performancebetween the hand-painted cathode and E/E cathodes.

Example 7

Electrochemical properties of the electrodes fabricated in Example 4were studied by cyclic voltammetry (CV), which was conducted on atwo-electrode membrane electrode assembly (made in Example 4) with apotentiostat (Solartron SI 1287, Corrware Software) at 20 mV/s over arange of 0 to 1.2 V. The anode worked as both the counter and referenceelectrodes. The electrochemical surface area (ECSA) of the electrodeswas determined from the hydrogen adsorption area from 0.1 to 0.4 V. Thefuel cell anode and cathode were supplied with H₂ at 40 sccm (standardcubic centimeters per minute) and N₂ at 18 sccm, respectively.Temperatures of the cathode, anode and fuel cell were held at 30° C. ThePt catalyst was assumed to have an average site density of 210 μC/cm².Using the Pt loading and maximum power density (H₂/air at ambientpressure), the platinum utilization (g of Pt in cathode/kW of fuel cellmax power) was calculated.

Several electrochemical properties of the electrodes are summarized inTable 1. The E/E electrodes had much higher ECSA than the hand-paintedelectrodes. The platinum utilization value for the E/E electrodes waslower than the target set by the U.S. Department of Energy (DOE 2012target) indicating that these electrodes meet DOE specifications. Thesuperior performance of fuel cells with E/E electrodes even with less Ptloading may be at least partially due to the reason that the E/Eelectrodes had ECSAs significantly higher than the hand-painted controlelectrode.

TABLE 1 Electrochemical properties of catalyst layers in electrodes Ptutilization at Loading, maximum power, Cathode mg/cm² ECSA, m²/gg_(Pt)/kW Hand-painted 0.42 53.2 0.71 E/E 0.022 121.3 0.059 E/E 0.052101.7 0.121 DOE 2017 target — — 0.125

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meanings of the terms inwhich the appended claims are expressed.

What is claimed is:
 1. A structure comprising: a polymer fiber structurecomprising fibers having a fiber diameter of less than 1 micron; and aplurality of catalytic material particles supported on the polymer fiberstructure.
 2. A structure as claimed in claim 1, where in the polymerfiber structure comprises aligned fibers.
 3. A structure as claimed inclaim 1, wherein the catalytic material has a particle size of less than50 nm.
 4. A structure as claimed in claim 1, wherein the catalyticmaterial has a particle size of less than 20 nm.
 5. A structure asclaimed in claim 1, wherein the catalyst loading is less than 0.1 mg ofcatalyst per square centimeter.
 6. A structure as claimed in claim 1,wherein the catalyst loading is less than 0.05 mg of catalyst per squarecentimeter.
 7. A structure as claimed in claim 1, wherein the catalystloading is less than 0.01 mg of catalyst per square centimeter.
 8. Anelectrode comprising the structure of claim
 1. 9. A fuel cell comprisingthe electrode of claim
 8. 10. A method for making nanofiber-nanoparticlenetwork to be used as electrode of a fuel cell, the method comprises thestep of: electrospinning a polymer-containing material onto a substratefor forming nanofibers on the substrate, and electrospraying acatalyst-containing material onto the polymer-containing material ornanofibers.
 11. The method of claim 10, wherein said electrospinning andsaid electrospraying are carried out simultaneously.
 12. The method ofclaim 10, wherein the polymer comprises at least one polymer selectedfrom the group consisting of Nafion, sulfonated poly(ether etherketone), sulfonated polyer(styrene-b-ethylene-r-butadiene-b-styrene),sulfonated poly(styrene), sulfonated poly(arylene ether) copolymer,sulfonated poly(styrene-b-isobutylene-b-styrene).
 13. The method ofclaim 10, wherein said polymer is a sulfonated tetrafluoroethylenes. 14.The method of claim 10, wherein said polymer-containing material is asolution.
 15. The method of claim 10, wherein said polymer-containingmaterial is molten polymer.
 16. The method of claim 10, wherein thecatalyst comprises at least one material selected from the groupconsisting palladium, platinum, gold, silver, nickel, rhodium,ruthenium, rhenium, osmium, iridium, iron, chromium, cobalt, copper,manganese, tungsten, niobium, titanium, tantalum, lead, indium, cadmium,tin, bismuth and gallium, as well as compounds and alloys of thesemetals.
 17. The method of claim 16, wherein said at least one fuel cellcatalyst is synthetic platinum on carbide derived carbon support. 18.The method of claim 16, wherein said catalyst-containing material is asolution.
 19. The method of claim 1, further comprising the step ofheating the materials.
 20. A catalyst for a fuel cell selected from thegroup consisting of platinum on carbide derived carbon support.